introduction

In the context of the Sumatra tsunami, this briefing document gives
information on what is a tsunami, why do they occur, what are the
results (wave size, speed, distance travelled) and the effects on
humanity and the environment. As well as an wide-ranging investigation
on tsunamis, this briefing document at abelard.org provides illustrations
and diagrams, together with numerous links to specialised websites.

[Tsunami means “harbour wave” in Japanese.]

a
tsunami strikes

This
is probably the main source tracking the Sumatra tsunami. An
animation of the first few hours only, so far. Bangladesh appears
to have escaped lightly, despite having 17 million people living less
than one metre above sea level. Another
animation of the first three hours. These show weaker waves going
North and South, than the waves going East and West. This is probably
why much of India and Bangladesh has been little affected.

Seismic diagram of the Sumatran quake of 26 December 2004.
Image credit: UNESCO

terrestrial
consequences of the sumatran undersea quake

“The shockwave shortened the period of our planet's rotation
by some three microseconds. [...] [NASA] estimate that Earth now
tilts by an extra 2.5 centimetres in the wake of the jolt.”
[Quoted
from nature.com]

“The earth is regularly deformed by the daily passage of
the sun and moon, raising a tide in the earth of about 20cm, and
the vibrations we saw on Boxing Day shortly after the earthquake
off the Indonesian coast were of a similar magnitude on our instruments.”
—
A fortnight after, “equipment was showing much of the planet
was still ringing like a bell.[...] "The main signal we now
see is a steady oscillation of a few parts in 10 billion of normal
gravity, which corresponds to about a millimetre of vertical motion
of the earth," ” [Quoted
from Herald Sun]

There is a good write-up from the overseers of the US Tsunami Service,
NOAA
and Tsunamis.

“The waves radiate outward in all directions from the disturbance
and can propagate across entire ocean basins. For example, in 1960
an earthquake in Chile caused a tsunami that swept across the Pacific
to Japan. Tsunami waves are distinguished from ordinary ocean waves
[1] by their great length between peaks, often
exceeding 100 miles in the deep ocean, and by the long amount of
time between these peaks, ranging from five minutes to an hour.
The speed at which tsunamis travel depends on the ocean depth. A
tsunami can exceed 500 mph in the deep ocean but slows to 20 or
30 mph in the shallow water near land. In less than 24 hours, a
tsunami can cross the entire Pacific Ocean.

“In the deep ocean, a tsunami is barely noticeable and will
only cause a small and slow rising and falling of the sea surface
as it passes. Only as it approaches land does a tsunami become a
hazard. As the tsunami approaches land and shallow water, the waves
slow down and become compressed, causing them to grow in height.
In the best of cases, the tsunami comes onshore like a quickly rising
tide and causes a gentle flooding of low-lying coastal areas.”

When a wave
hits the shore, it tends to act like a motorway pile-up
to varying degrees. This is called the ‘run
up’. A typical beach has a run up of
3; that is, a 5 foot wave will rise to 15 feet on
beaching (three times the starting height of the wave).

This will, of course, depend on the beach slope. Naturally, a steep
slope causes greater run up than a gentle beach. Even greater run
ups would be expected in an inlet like a river or delta. Regions with
run ups of approximately 40 are known.

Various types of tsunami give different patterns
of waves formation and propagation, this is an area of study
that is rapidly developing and involves computer modelling, real-world
and experimental data. Study of the Shoemaker-Levy comet, which crashed
into Jupiter in 1994, has resulted in increased confidence in computer
models.

A common convenient, if grim, measure of the energy is by megatons
of TNT, or numbers of Hiroshima-type bombs. Hiroshima was rated at
about 15,000 tons of TNT.

sumatra
tsunami present and past

“Many who escaped death in what was possibly the deadliest
tsunami in more than 200 years now face hunger and disease. The
United Nations mobilized what it called the biggest relief operation
in its history [...] ”

Note that previously gray roads and light-coloured compounds are now
brown from floodwater.

The epicentre of the 9.0
Richter undersea quake that formed the tsunami of 26 December 2004
was about 155 miles from the north-western coast of Sumatra.

“A tsunami in 1883 at Krakatoa, off southern Sumatra, killed
36,000 and one in the South China Sea in 1782 killed 40,000, according
to the U.S. National Geophysical Data Center.” [Quoted from Reuters]

“The deadly Asian earthquake may have permanently accelerated
the Earth's rotation -- shortening days by a fraction of a second --
and caused the planet to wobble on its axis [...] ” [Quoted from
tvnz.co.nz]

Present estimates suggest that this earthquake had the energy of more
than 10,000 Hiroshima bombs, that is between 150 to 200 million tons
of TNT

The wave went up to 5 kilometres inland.

The last large tectonic event also on this fault, that resulted in
an earthquake, was in 1843. Thus the pressures and forces have had 162
years to build up. This is why the sea-quake was so powerful (9.0 on
the Richter scale). The tectonic rupture is reported as being 1,200
kilometres long, and as occurring as three events, within seconds of
each other. The first was to the west of North Sumatra, initiating two
more slips further north.

The largest tsunami recorded measured 210 feet, about 18 stories above
sea level, when it reached Siberia's Kamchatka Peninsula in 1737 - probably!
[33 feet has been reported so far for the Sumatran tsunami.]

“The most damaging tsunami on record before 2004 was the one
that killed an estimated 40,000 people in 1782 following an earthquake
in the South China Sea. [...] In northern Chile more than 25,000 people
were killed by a tsunami in 1868.

“The Pacific is by far the most active tsunami zone, according
to the U.S. National Oceanic and Atmospheric Administration (NOAA).
But tsunamis have been generated in other bodies of water, including
the Caribbean and Mediterranean Seas, and the Indian and Atlantic Oceans.
North Atlantic tsunamis included the tsunami associated with the 1775
Lisbon earthquake that killed as many as 60,000 people in Portugal,
Spain, and North Africa. This quake caused a tsunami as high as 23 feet
(7 meters) in the Caribbean.”
[Quoted from National
Geographic.com]

lack
of preparation

“LOS ANGELES: US officials who detected a massive earthquake
off Asia's coast tried frantically to warn the deadly wall of water
was coming, the head of the Pacific Tsunami Warning Centre has said.

“But there was no official alert system in the region because
such catastrophes only happen there about once every 700 years, said
Charles McCreery, director of the National Oceanic and Atmospheric Administration's
Centre in Honolulu.

“ "We tried to do what we could," McCreery said. "We
don't have contacts in our address book for anybody in that part of
the world."

“Within moments of detecting the quake, McCreery and his staff
were on the phone to Australia, then to US Naval officials, various
US embassies and finally the US State Department.”

“ US officials are now trying to help officials in the region
set up some sort of informal warning system and feeling badly that more
couldn't have been done, McCreery said.

“ "It took an hour and a half for the wave to get from the
earthquake to Sri Lanka and an hour for it to get...to the west coast
of Thailand and Malaysia," he said. "You can walk inland for
15 minutes to get to a safe area." ” [Quoted from stuff.co.nz]

Reports [30.12.04] are now giving estimates of well over 100,000 dead
- approximately 1/60,000 of the world population, or the population of
a UK town like Oxford (135,000); and maybe 5 millions washed out (getting
on for 1/1000 of the world’s population).

warning
signs

Most people think that a tsunami is one giant wave,
like the ones they see surfers sliding down. A tsunami
is more like a river bore, a “wall of water”,
than a surfer’s wave tastefully curling over.

The first water wall is followed by other waves that can occur over a
period of hours. In all, a tsunami is more like a coastal flood. Unless
the observer is close to the originating cause,
they will probably receive little warning of the event; that is, there
will be no associated change in the weather.

On a beach, the only regularly reported clear
warning sign that an dangerous sea event is going
to happen soon is

all the sea retreats fast
and far, maybe about 10 metres,
giving an unexpected and very sudden low tide.

Another warning sign is to see a relatively high line of frothing,
white water on or near your sea horizon. That is the tsunami
wave coming in. Run!

Remember, the water that retreated abnormally will not stay retreated,
it will return and, even it if it is not of tsunami proportions, will
be a considerable mass of water.

If you are lucky enough to hear about a large sea-based earthquake in
time, get away from the water’s edge - now, and stay away. Of course,
major earthquake-related tsunami events occur rarely, but they are not
the sort of phenomenon in which you would wish to participate.

As you will know, water en masse can often be dangerous or,
at the very least, a nuisance. Every year, a few people are killed by
flash floods, breaking dams, or even a fast-rising tide when they have
become trapped on a beach without local knowledge.

In more naïve times, during a business lunch-hour, I sat on a
rock, reading and sunning myself, alongside a fairly broad channel.
I had the special treat of watching a fairly large naval ship moving
down the channel. I then went back to my reading. Five or ten minutes
later, I was up to my armpits in water and with my clothes floating
away. I didn’t know that a ship’s wash could do that —
then....

If you feel an earthquake or see a landslide when on the coast, at a
beach, again think “tsunami” and move. Tsunamis are a likely
consequence of an earthquake or a landslip (see next section).

It is possible that some animals may also behave abnormally, particularly
some large mammals and some birds known to be able to sense very low-frequency
sounds. Associated with earthquakes, infrasound
(below human hearing, less than ~20 Hertz) travels through the the earth
at several times the speed of sound in air, very much faster than tsunamis.
Animals try to escape that which frightens them. This might provide a
warning indicator, though difficult to verify.

“"From the size of the earthquake, it
is likely that the average displacement on the fault
plane was about fifteen meters. The sea floor overlying
the thrust fault would have been uplifted by several
meters as a result of the earthquake.”

Islands are being reported as moved laterally
in the area by several metres have probably not
moved sideways, but been upthrusted thus changing
their coast lines.

The Richter
scale is a measure, or derivation, of the
energy released by the quake.

A large tsunami requires an undersea quake of
at least 7.5 on the Richter scale to form. A regional
or local tsunami may be formed by a quake of about
6.5.

The measurement of earthquakes using a scale
was only developed in 1935, by Charles Richter
at the California Institute of Technology.

Earthquake Severity
- Richter scale

scale

effects

approx.
annual frequency

Less than 3.5

Generally not felt, but recorded.

millions

3.5 - 5.4

Often felt, but rarely causes
damage.

5.5 - 6.0

At most slight damage to well-designed
buildings.
Can cause major damage to poorly constructed buildings
over small regions.

6.1 - 6.9

Can be destructive in areas
up to about 100 kilometres across where people
live.

150

7.0 - 7.9

Major earthquake.
Can cause serious damage over larger areas.

15

8 and over

Great earthquake.
Can cause serious damage in areas several hundred
kilometres across.

1

The largest earthquake ever
recorded by this method occured on 22 May
1960, reading 9.5 on the Richter scale, badly damaging
much of Chile. That quake also generated a 10m/30ft
tsunami that washed away whole villages in Chile
and killed 61 in several hundred miles away in Hawaii.

11 March 2011: An
earthquake, estimated at 8.9 on the Richter scale,
hits mid and north-eastern Japan. An associated
tsunami has reached across the Pacific Ocean to
the western coasts of Canada and the United Staes
of Amerca.

The
JMA [Japanese Meteorological Agency] Seismic Intensity
Scale lists the local effects of different
levels of seismic intensity on people, buildings,
services and ground.

JMA Seismic Intensity
Scale

scale

effects

0

Imperceptible to people.

1

Felt by only some people in
the building.

2

Felt by most people in the
building. Some people awaken.

3

Felt by most people in the
building. Some people are frightened.

4

Many people are frightened.
Some people try to escape from danger. Most sleeping
people awaken.

5 lower

Most people try to escape
from a danger.Some people find it difficult to
move.

5 upper

Many people are considerably
frightened and find it difficult to move.

tectonic
subduction

“What happened in Asia may give a vivid
demonstration of the geologic future of the Pacific
Northwest. For hundreds of years, these subduction
zone plates remain locked, releasing little of their
tension. The plate which is being subducted is forced
down, while the plate above bulges upwards. Then,
in a few minutes of violence every few centuries,
the forces are released.[1] The
upper plate moves seaward, and a massive tsunami
can be produced along with catastrophic destruction
from earthquake shaking.

“In the case of the Cascadia Subduction Zone,
you could have an area of ocean sea floor that's
50 miles wide and 500-600 miles long suddenly snap
back up, causing a huge tsunami," Goldfinger
said. "At the same time, we could expect some
parts of the upper, or North American plate to sink
one to two meters. These are massive tectonic events.
Subduction zones produce the most powerful earthquakes
and tsunamis in the world.” [Quoted from oregonstate.edu]

“Description: The majority of earthquakes
and volcanoes around the world occur at the intersection
of plate boundaries. This diagram shows the subduction
of an ocean plate underneath a continental plate.
Earthquakes are caused by the two plates moving
relative to each other, and volcanoes are formed
when ocean crust, forced under the lighter continental
crust, melts and then rises to the Earth's surface.
The Andes have been forming over the past 170
million years as the Nazca Plate lying under the
Pacific Ocean has forced its way under the South
American Plate and pushed up its western edge.
The subduction of one plate under the other has
given rise to a number of volcanoes that dot the
western edge of the mountain range.”

Ash greater than one centimetre thick fell on more than 500,000
square kilometres of Indonesia and the Java Sea.

Before the eruption Tambora was a 4,300-metre-high stratovolcano;
following the eruption 1,400 metres of the summit cone were missing
and in its place was a collapsed caldera measuring six by seven
kilometres wide and one kilometre deep.

About 10,000 people were killed by the explosive eruption and
the tsunamis caused by massive pyroclastic flows entering the
sea.

Agricultural losses from the thick ash deposits resulted in
famine and disease, leading to an additional 82,000 deaths.

Fifty cubic kilometres is approximately 12
cubic miles [1 cubic km ~ 4.168 cubic miles], the amount a 500-yard
diameter asteroid could be expected to throw into the air, assuming
a 50,000 mph impact.

Krakatoa (Also in Indonesia)

The second largest eruption of the 19th century also occurred
in Indonesia. Krakatoa (or Krakatau), a compound volcano on a small
uninhabited island between Sumatra and Java, erupted explosively
on 26-27 August, 1883. The eruption was similar to the Tambora outburst
but smaller, involving only about 18 cubic kilometres of magma erupted
in Plinian ash clouds and pyroclastic flows. Krakatoa was a much
smaller volcano than Tambora, and when the eruption had emptied
its magma chamber, it collapsed to form a caldera that was partly
below sea level.

Twenty-three square kilometres of the island of Krakatoa disappeared,
and where a volcanic peak 450 metres high once stood was water as
deep as 275 metres. The largest explosion on the morning of August
27 produced an ash cloud that was reported to have reached 80 kilometres
high, and the detonation was heard in Australia, 4,800 kilometres
away. A tsunami, over 30 metres high, followed the explosion and
apparent caldera collapse, killing about 36,000 people on the adjacent
shores of Java and Sumatra.

asteroids
and meteors

About one third of the world is land, so an asteroid or meteor impact
is more likely to occur at sea.

Large asteroids also form a ‘chimney’ in the atmosphere,
by which large quantities of material may be sucked into the atmosphere.
Thus the effect of an asteroid can be greater than a major volcanic
eruption. The Tambora event in 1815 (also
in Indonesia) is associated with “the
year with no summer”, causing major crop failures around
the world.

The effects of large amounts of water being vapourised and ejected into
the upper atmosphere is yet unknown. Remember, water vapour is a major
greenhouse gas.

There are approximate frequencies of asteroid/meteor impact by size,
as calculated from past data. The force of an asteroid varies with size
and speed of impact. It has effects similar to a pebble dropped in a pond,
but can be a damned big, fast pebble.

approximate
probabilities of various levels of strike impact

impact diameter

kinetic energy

area devastated

average interval (years) between asteroid impacts

(m)

Mt
TNT

sq km

anywhere
on Earth

on
a ‘city’

on inhabited
region /
expected death toll

50

10

1900

100

30,000,000

900 yr
1,000,000

100

75

7200

1000

70,000,000

8000 yr
3,000,000

200

600

29,000

5000

90,000,000

30,000 yr
14,000,000

500

10,000

70,000

40,000

290,000,000

180,000 yr
30,000,000

1 km

75,000

200,000

100,000

260,000,000

290,000 yr
60,000,000

2 km

1,000,000

-

1,000,000

-

1 ,000,000 yr
1.5 billion

based on the impact of a stony asteroid,
assuming velocity=20km/s [approx. 72,000 kph/45,000
mph], density=3 g/cc

Table derived from section
on asteroids. You want to study asteroids further? Go to the linked
site!

Ordinary ocean and lake waves
are formed by the friction of wind on the water.
Thus, they are kicked up according to the intensity
of the wind. Waves are
not a movement of water forward, the movement only
is transmitted through the water, molecule to molecule.
Hence, the speed of a tsunami can be much greater,
because of the much greater generating forces. As
with the waves and also with a tsunami, it is the
wave that travels and not the water.

A tectonic plate slip can be a very fast, explosive
event. Think in terms of bending a stick, where
the tension gradually increases and then suddenly,
the stick snaps back. If you want to see this happen,
push a fairly thin green branch at an angle, up
against a brick or concrete wall and the stick will
bend, and then at some point the branch will slip
and spring back. (If
you are youthful, don’t do this without supervision;
and if you’re more experienced, take precautions,
because the energy when the branch snaps back can
be dangerous.) The speed of the event triggering
the tsunami will determine the speed of the propagating
wave, and the size/volume of the event will determine
the size of the wave.

Windows Media
Player

Of course, all earthquakes are not equal. They
happen at various depths and, to be sure, some
happen on land. The depth of water where the event
occurs can also differ greatly. In fact, it often
takes considerable time before all the data is
gathered and analysed, and a clear idea of the
position and power of the event is assessed.

The present estimate for the Chilean earthquake
[February 2010] is it occured at a depth of 21
miles, whereas the Haitian earthquake [January
2010] is estimated to have been at about 8 miles
depth.

With some earthquakes, much of the energy will
dispersed sideways. In others, the snapback
- that lifts and drops vast masses of water -
may be dominant.

how
fast does a tsunami travel - the chilean
event

The 27th February, 2010 Chilean main event,
at 06:34:17, was originally assessed at
8.8 on the Richter scale.

This much milder tsunami event, relative
to the Sumatrian
’quake-generated tsunami, took
about 12 hours to reach Hawaii and about
13 to 14 hours to reach New Zealand. It
was detected in Japan after a little over
24 hours, a distance of about 17,000 km
or over 10,000 miles.

So the tsunami appears to have been propagated
at approximately 700 kph or 400 mph.